Thin-film transistor structure for liquid crystal display having black matrixes connected to a gate line

ABSTRACT

A thin-film transistor structure having a storage-capacitor-on-gate and a black matrix for manufacturing a liquid crystal display is disclosed. A metal layer is deposited and patterned as a black matrix on a glass substrate of the thin-film transistor plate. An insulating layer having a contact hole for contacting the black matrix is formed over the surface of the black matrix and the substrate. An inverted thin-film transistor having a metal gate on the bottom is then fabricated on top of the insulating layer. The thin-film transistor controls an ITO pixel electrode of the liquid crystal display. A gate line including the metal gate of the thin-film transistor is formed over and above a space between two adjacent black matrixes. The gate line is connected to one of the two black matrixes by the contact hole. The other black matrix serves as a light shield element of the ITO pixel electrode. The technique is also applicable to the manufacturing of a non-inverted thin-film transistor having a metal gate on the top.

This is a division of application Ser. No. 08/785,482, filed Jan. 17,1997 now U.S. Pat. No. 5,879,959.

FIELD OF THE INVENTION

The present invention relates to a liquid crystal display devicestructure as well as its manufacturing method, and more specifically tothe fabrication of a thin-film transistor having a storage capacitor ongate and a black matrix for a liquid crystal display device.

BACKGROUND OF THE INVENTION

Thin-film transistor (TFT) liquid crystal displays (LCDs) have beenwidely used in areas such as personal notebook computers, portabletelevisions and video camera monitors. In conventional TFT LCDs, theback-light consumes most of the power. In order to reduce the powerconsumption of the LCD devices, there are two approaches commonlyadopted. One is improving the efficiency of the back-light and the otheris increasing the transmission of the light through the liquid crystalcells.

Many different technologies of manufacturing TFT-LCD devices withvarious structures have been presented to improve the back-lightefficiency. U.S. Pat. No. 5,478,611 of T. Hashimoto discloses a glasssubstrate for LCDs in which unnecessary light of the region other thanpixel electrodes can be completely shielded, the TFT optical leakcurrent can be totally suppressed, and the light reflection at the blackmatrix portion can be almost fully depressed. M. Katayama et. al.presented "A 10.4 in. Diagonal Full-Color TFT-LCD with New Self-aligneda-Si TFTs for OHP System" in pages 243-246 of Conference Record of the1991 International Display Research Conference. The light degradation ofthe LCD performance has been suppressed by new self-aligned a-Si TFTsand a new driving method presented in the article.

The light transmission through the liquid crystal cells depends on theaperture ratio of a TFT-LCD device. Much research and development workhas been dedicated to obtaining a high aperture ratio for TFT-LCDdevices. N. Takahashi presented a structure that achieves a 35% aperturein "A High-Aperture Ratio Pixel Structure for High-Density a-Si TFTLiquid-Crystal Light Valves" of SID 1993 DIGEST, pp. 610-613. "AHigh-Aperture TFT-LCD with Shield-Electrode Structure" of SID 1993DIGEST, pp. 739-742 presented by T. Ueda shows that a 58% aperture hasbeen developed. T. Kitazawa et. al. further shows an ultra-high-apertureratio of 70% by using a shield-electrode structure and astorage-capacitor-on-gate structure in "A 9.5-in. TFT-LCD with anultra-High-Aperture-Ratio Pixel Structure" of SID 1994 DIGEST, pp.365-368. An analytical investigation of the aperture ratio on highTFT-array structure has been discussed by K. Suzuki in "High-ApertureTFT Array Structures" of SID 1994 DIGEST, pp. 167-170.

It is clear that there is a strong need in achieving a high apertureratio for the TFT-LCD devices. Various designs and techniques in formingthe black matrix structure in TFT-LCDs have been studied to increase theaperture ratio and improve the back-light efficiency.

SUMMARY OF THE INVENTION

The present invention has been made according to the need of improvingback-light efficiency for a TFT-LCD device as described above. A primaryobject of this invention is to provide a new TFT structure having astorage-capacitor-on-gate and a black matrix integrated together on theTFT plate for the LCD device. Therefore, the invention is to provide aTFT-LCD device having good performance and a high aperture ratio. Asecond object of this invention is to provide manufacturing methods forfabricating such TFT-LCD structures.

In the present invention, a storage-capacitor-on-gate structure and ablack matrix structure are combined together in the TFT-LCD device. Ablack matrix is formed on top of the glass substrate of the TFT plate.An insulating layer is deposited above the black matrix. A thin-filmtransistor is then fabricated on the insulating layer for controlling anITO pixel electrode of the LCD device. An LCD display comprises an arrayof thin-film transistors and an array of black matrixes on the sameplate. A gate line including the gate of a thin-film transistor isformed above and over a space between two adjacent black matrixes. Oneblack matrix serves as a light shield element of the ITO pixel electrodecontrolled by the transistor. The other black matrix that shields theITO pixel electrode controlled by the next transistor on the adjacentgate line is connected to the gate line. When the TFT-LCD device is inoperation, only the selected gate line is driven. The rest of the gatelines are connected to the common electrode. A storage capacitor isformed by the pixel selected and the black matrix or the gate line underthe pixel. Therefore, a structure of storage-capacitor-on-gate is formedfor the TFT.

The black matrix of this invention is formed below the thin-filmtransistor. There is no need to manufacture another black matrix on thecolor filter plate. The aperture ratio of the TFT-LCD is increased. Theinvention also decreases the chance of the short between the gate lineand the common line because they are manufactured in different layers.In addition, the gate resistance is also reduced because of its parallelconnection to the black matrix. It eliminates the need of forming doublelayers for the gate. The black matrix, which highly reflects the lightsource, avoids heating of the transistors. Therefore, the TFT-LCDs ofthis invention are very appropriate for application in projectors wherethe transistors will be strongly illuminated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of the glass substrate, the blackmatrix layer and the first insulating layer of the TFT plate of thepresent invention.

FIG. 2a shows the plan view of a portion of the LCD devices having ablack matrix pattern of the present invention.

FIG. 2b shows the plan view of a portion of the LCD devices having analternative black matrix pattern of the present invention.

FIG. 3 shows cross-sectional views of FIG. 2a comprising aback-channel-etching type inverted staggered TFT.

FIG. 4 shows cross-sectional views of- FIG. 2b comprising aback-channel-etching type inverted staggered TFT.

FIG. 5 shows a cross-sectional view of an etching-stop type invertedstaggered TFT used in the present invention.

FIG. 6 shows a cross-sectional view of a staggered TFT used in thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIG. 1, a glass substrate 1 is used as the startingsubstrate of the TFT plate of the present invention. A first layer ofmetal is first deposited on the substrate. The preferred material of themetal layer is chromium and its thickness is approximately 1000 to 2000Angstroms. Similar materials such as molybdenum, aluminum, tungsten,tantalum, or titanium may also be used. The metal layer is patterned andetched by a conventional photolithographic technique and an etchingprocess to form black matrix patterns 2 as shown in FIG. 1. The planviews of the black matrix array are shown in FIGS. 2a and 2b. A firstinsulating layer 3 is deposited on top of the glass and the blackmatrix. The insulating layer is typically made of SiNx or SiOx and thethickness is about 500 to 4000 Angstroms.

Two types of patterns can be adopted for the black matrix as shown inFIGS. 2a and 2b. The gate line 10, the data line 16 and the black matrixarray 2 are shown in the figures. The plan view of a contact hole 4 thatconnects the gate line 10 and the black matrix 2 is also shown. Thecross sectional views across lines A-B and A-C of FIG. 2a and 2b areshown in FIGS. 3 and 4. As will be explained later, A-B shows a crosssection of a thin-film transistor and A-C shows how the black matrix isconnected to the gate line of a next thin-film transistor. It should benoted that the black matrix array 2, the gate line 10, the data line 16and the contact hole 4 of FIG. 2a (or FIG. 2b) correspond to the blackmatrix pattern, the second metal layer, the third metal layer, and thecontact hole of FIG. 3 (or FIG. 4) respectively. Therefore, the samenumerals are used in FIG. 3 (or FIG. 4) for the correspondingstructures. The width of the black matrix pattern line can be designedaccording to the requirement of the TFT-LCD device.

The thin-film transistor is fabricated on the surface of the firstinsulating layer 3. Two different basic structures have been widely usedin the industry for manufacturing the thin-film transistors of an LCDdevice. One uses an inverted thin-film transistor structure thatcomprises a gate at the bottom of the transistor. The other uses athin-film transistor structure that comprises a gate on the top of thetransistor. Both inverted and non-inverted thin-film transistors can beeither staggered or coplanar. For a coplanar structure, the gate and thedrain/source are formed on the same side of an island structure. For astaggered structure, they are on a different side. The detailedstructure, manufacturing process and materials used in these basicstructures have been well known to one skilled in the art. Therefore,only the details that are necessary for understanding the currentinvention will be described.

FIG. 3a shows a cross sectional view of the present invention along theline A-B of FIG. 2a. The thin-film transistor of this embodiment is aback-channel-etching (BCE) type inverted staggered TFT. FIG. 3b showsthe cross sectional view along the line A-C of FIG. 2a. Beforefabricating the thin-film transistor, the first insulating layer 3described above is patterned and etched using conventional techniques toform a contact hole 4 for contacting the black matrix as shown in FIG.3b. The contact hole is located below the place where the gate line ofthe TFT is formed as illustrated in FIG. 2a and FIG. 3b. A second metallayer 10 is then formed on top of the first insulating layer as the gateline. The contact hole is also filled by the second metal layer 10. Thesecond metal layer is patterned and etched appropriately to cover thecontact hole and to form the gate metal for the thin film transistor. Itshould be noted that each black matrix is connected to the gate line ofthe next thin-film transistor by means of the contact hole as shown inFIG. 2a and FIG. 3b.

The remaining process of forming the thin-film transistor can becompleted as customary. A gate insulating layer 11 of silicon nitride isdeposited on the device to cover the gate metal and the first insulatinglayer. A layer of amorphous silicon (a-Si) 12 and a layer of n+-typeamorphous silicon 13 are deposited on top of the gate insulating layer.Both the a-Si layer and the n+-type a-Si layer are patterned and etched.An ITO pixel electrode 14 is then formed above the gate insulating layer11. The next step comprises the formation of a contact hole forconnecting the metal gate line to the peripheral circuit of the LCDdevice that is not shown in FIG. 3. A third metal layer 16 is thendeposited and patterned on the surface to form the source and drainmetal. Finally, the n+-type a-Si layer is etched.

Similarly, FIG. 4 shows the cross sectional views along lines A-B andA-C of FIG. 2b. Although the black matrix pattern is slightly differentfrom that shown in FIG. 2a and FIG. 3, the principle of operation andthe process of manufacturing the thin-film transistor are identical.FIG. 4a shows a cross sectional view of the present invention along theline A-B of FIG. 2b. The thin-film transistor shown in FIG. 4a is aback-channel-etching (BCE) type inverted staggered TFT. FIG. 4b showsthe cross sectional view along the line A-C of FIG. 2b. As can be seenin FIG. 4a, the black matrix pattern covers a large area above which thethin-film transistor is fabricated. The first insulating layer 3 ispatterned and etched to form a contact hole 4 for contacting the blackmatrix as shown in FIG. 4b. The contact hole is located below the placewhere the gate line of the TFT is formed as illustrated in FIG. 2b andFIG. 4b. A second metal layer 10 is then formed, patterned and etchedappropriately as the gate line to cover the contact hole on top of thefirst insulating layer. The contact hole is also filled by the secondmetal layer 10. As described for FIG. 3 before, the black matrix of thisinvention should be connected to the gate line of the next thin-filmtransistor by means of the contact hole as shown in FIG. 2b and FIG. 4b.

The following process of forming a gate insulating layer 11 of siliconnitride, a layer of amorphous silicon (a-Si) 12, a layer of n+-typeamorphous silicon 13, and an ITO pixel electrode 14 is completed ascustomary. The next step comprises the formation of a contact hole forconnecting the metal gate line to the peripheral circuit of the LCDdevice that is not shown in FIG. 4. Finally, a third metal layer 16 isdeposited and patterned to form the source and drain metal, and then+-type a-Si layer is etched.

According to this invention, each black matrix is connected to the gateline of its next TFT as shown in FIG. 2a. When the TFT-LCD device is inoperation, only the selected gate line is driven. The rest of the gatelines are connected to the common electrode. A storage capacitor isformed by the pixel selected and the black matrix or the gate line underthe pixel. A structure of storage-capacitor-on-gate is thus formed forthe TFT. It should be noted that the driving method for astorage-capacitor-on-gate structure has to be adopted for the LCD deviceof this invention. The storage capacitor increases the total capacitancefor the LCD device. Therefore, the decay of the display signal isdecreased.

The aperture area of an LCD device is the total area excluding both theblack matrixes and the gate lines that mask the light source during theLCD operation. The black matrix is fabricated below the TFT in thepresent invention, the aperture ratio is increased because no blackmatrix needs to be formed on the color filter plate. Although sometimesa light shield may still be formed on the color filter plate to reducethe light effect to the a-Si layer, the size of the light shield istypically much smaller than the total size of the black matrix and thegate line. The aperture ratio will not be reduced due to the lightshield. The gate resistance is also reduced due to the connection to theblack matrix. Therefore, a double layer gate commonly used for reducingthe resistance is not required. The possibility of short between thegate and the common line is minimized because they are in differentlayers. The invention also decreases the flicker of the display.

The thin-film transistor can also be an etching-stop type invertedstaggered TFT as shown in FIG. 5. Similar to the process described forthe embodiment shown in FIG. 3, the first insulating layer 3 ispatterned and etched using conventional techniques to form a contacthole 4 for contacting the black matrix before fabricating the thin-filmtransistor. The second metal layer 20 is formed on top of the firstinsulating layer, patterned, and etched to form the gate metal that isconnected to the black matrix through the contact hole.

A conventional method of fabricating the etching-stop type invertedstaggered TFT can be used to complete the TFT. A gate insulating layer21, an a-Si layer 22 and a SiNx layer 23 are formed above the gate metallayer and the first insulating layer. The SiNx layer 23 is patterned andetched. After etching, the remaining SiNx covers an area above the gatemetal 20. A layer of n+-type a-Si 24 is then formed on top of theremaining SiNx 23 and the a-Si layer 22. Both n+-type a-Si 24 and a-Si22 layers are patterned and etched. An ITO pixel electrode 25 is thenformed on the open area above the gate insulating layer. Contact holefor connecting the gate line to the peripheral circuit is formed. Thethird metal layer 27 is then deposited and patterned on the surface toform the source and drain metal.

FIG. 6 shows the cross sectional view of an alternative embodiment ofthis invention. The thin-film transistor of this embodiment is astaggered TFT. In this embodiment, A conventional process can be used tofabricate the first several layers for the TFT. An ITO pixel electrode30 is first formed on top of the first insulating layer 3 of the presentinvention. A source/drain metal layer 31 is deposited, patterned andetched. A n+-type a-Si layer 32, an a-Si layer 33, and a gate insulatinglayer 34 are appropriately formed as in the conventional process formanufacturing a staggered TFT. A contact hole is then fabricated forconnecting the data line to the peripheral circuit of the LCD device. Inthe present invention, an additional contact hole is also fabricated atthe same time for contacting and connecting to the black matrix.Finally, a gate metal layer 37 is deposited and patterned as shown inFIG. 6.

Although only the preferred embodiments of this invention were shown anddescribed in the above description, it is requested that anymodification or combination that comes within the spirit of thisinvention be protected.

What is claimed is:
 1. An array of inverted thin-film transistors andblack matrixes for a liquid crystal display comprising:a glasssubstrate; a metal layer deposited on said glass substrate, said metallayer being patterned and etched to form an array of black matrixes,each of said black matrix being separated from its adjacent black matrixby a space; an insulating layer formed above the surface of said arrayof black matrixes and said glass substrate, said insulating layer beingpatterned and etched to form an array of contact holes for contactingsaid array of black matrixes; and an array of inverted thin-filmtransistors fabricated on the surface of said insulating layer, eachtransistor of said array of transistors controlling an ITO pixelelectrode and having a bottom gate line formed above the space betweentwo black matrixes, both black matrixes having a part below andoverlapping the gate line, one of the two black matrixes being connectedto the gate line through one of said contact hole array, and the otherof the two black matrixes serving as a light shield element for the ITOpixel electrode.
 2. The array of inverted thin-film transistors andblack matrixes for a liquid crystal display according to claim 1,wherein said thin-film transistors are back-channel-etching typeinverted staggered thin-film transistors.
 3. The array of invertedthin-film transistors and black matrixes for a liquid crystal displayaccording to claim 1, wherein said thin-film transistors areetching-stop type inverted staggered thin-film transistors.
 4. The arrayof inverted thin-film transistors and black matrixes for a liquidcrystal display according to claim 1, wherein said thin-film transistorsare inverted coplanar thin-film transistors.
 5. An array of thin-filmtransistors and black matrixes for a liquid crystal display comprising:aglass substrate; a metal layer deposited on said glass substrate, saidmetal layer being patterned and etched to form an array of blackmatrixes, each of said black matrix being separated from its adjacentblack matrix by a space; an insulating layer formed above the surface ofsaid array of black matrixes and said glass substrate; and an array ofthin-film transistors fabricated on the surface of said insulatinglayer, each transistor of said array of transistors controlling an ITOpixel electrode and having a top gate line formed on top of saidinsulating layer above the space between two black matrixes, both blackmatrixes having a part below and overlapping the top gate line, one ofthe two black matrixes being connected to the top gate line through acontact hole fabricated through said insulating layer, and the other ofthe two black matrixes serving as a light shield element for the ITOpixel electrode.
 6. The array of thin-film transistors and blackmatrixes for a liquid crystal display according to claim 5, wherein saidthin-film transistors are staggered thin-film transistors.
 7. The arrayof thin-film transistors and black matrixes for a liquid crystal displayaccording to claim 5, wherein said thin-film transistors are coplanarthin-film transistors.